Grains and Byproducts
The primary nutritional contribution of grains is carbohydrate. Whole grains contain 62% to 72% starch, which is about 60% to 70% digestible by warmwater fish but less than 40% digestible by salmonids. Heating increases digestibility 10% to 15%. Starch in grains is a valuable binding agent in steam pelleted and extruded fish feeds.
Yellow corn is the most widely fed grain in North America. It is low in protein, and the protein is poor in amino acid balance. It contains approximately 20 mg/kg to 30 mg/kg of the yellow-pigmented xanthophylls (leutin and zeaxanthin), which impart undesirable yellow color in certain areas of the flesh of white fleshed fish. Approximately 11 mg of xanthophylls/kg feed can be used in catfish feeds without adverse effect on pigmentation of the flesh. Corn gluten meal contains 40% to 60% protein and is a good source of methionine; however, it is a highly concentrated source of xanthophylls (200-350 mg/kg). Corn byproducts from distilleries and breweries are relatively high in protein (26-28%) and fat (8-10%), but the protein is low in lysine.
Rice bran, which usually includes polishing, contains approximately 12% protein, 12% fiber, and 12% fat. It is often available at reasonable cost in developing countries where other ingredients may be prohibitively expensive. It is a good nutrient source, but does not pellet well because of the high fat and fiber contents.
Wheat is usually too valuable for human foods to feed to fish; however, small amounts of ground wheat are often used in fish pellets because of binding quality. Wheat gluten (protein) is an excellent binder. Wheat bran and middings are commonly used in salmonid feeds because they contain mor protein and less starch than whole grains.